When you get to know me
I'll be long time gone.

If you ever saw me
I was someone else back then.

I have two younger siblings
and the middle one is your best peer.

The youngest one you haven't met.
And when you meet,
he'll disappear.


The question is, who are the mysterious brothers?

I guess that:

It is the past, the present and the future.

I reached this by guessing what would reasonably fit in the given text, and noting that:

All the narrative are about the time of when something happens.

When you get to know me
I'll be long time gone.


When you know the past (the one who refers himself/herself as "me"), it is something that do not exists anymore.

If you ever saw me
I was someone else back then.


When you saw the past, it was not the past, it was the present.

I have two younger siblings
and the middle one is your best peer.


The past has two younger siblings, the present and the future. The present is your best peer.

The youngest one you haven't met.
And when you meet,
he'll disappear.


When you meet the future, he will be the present instead, and immediately become the past.

They are:

YESTERDAY, TODAY AND TOMORROW

And you are:

YESTERDAY

Explaination

When you get to know me I'll be long time gone.

When a day is "yesterday" it is forever "gone".

If you ever saw me I was someone else back then.

"Yesterday" (as a day) changed every day. i.e. for every day there is a different "yesterday".

I have two younger siblings

"younger" means they first occur later in time (fits with "today" and "tomorrow").

and the middle one is your best peer.

"today" is the one we are always "with".

The youngest one you haven't met. And when you meet, he'll disappear.

"tomorrow" is the "youngest". When we encounter "tomorrow" (i.e. arrive there in time), it has become "today".

• Wow, two correct answers within two minutes of the question being posted! Impressive performance from both of you. Mar 8 '15 at 12:43